A normal method to adjust AC motor shaft rotation speed is to adjust the supplying voltage magnitude and frequency by a frequency converter (FC). Prevailing FC technology is called PWM (pulse width modulation), wherein the device is comprised of a rectifier for mains voltage rectification, a filtered DC voltage intermediate power bus and an inverter bridge for generating the output voltage. The output voltage consists of pulses formed by fast power electronic switches, normally IGBTs (insulated gate bipolar transistors). A similar inverter unit as used in motor side may also be used in mains side e.g. in cases where the motor may work in generator mode and the generated power is desirable to supply back to the mains. This kind of regenerative system is called as regenerative AC/AC converter in this document.
An external power supply/storage element may be connected in the DC intermediate power bus of a FC e.g. for a redundant power supply in case of mains power malfunction. This kind of power element may be e.g. a battery, a photovoltaic power plant, a fuel cell generator etc.
A problem caused by a frequency converter, especially when comprised a regenerative mains bridge connected to a grounded network, may be the common mode voltage in the DC intermediate power bus. The fast switching speed of an IGBT, in combination with a possible resonance in connecting cables may generate hazardous voltage spikes across the insulation from the external power supply/storage element to ground, thus shortening the lifetime of the insulation layers.
The problem may be avoided by using a dedicated transformer between the mains power grid and the frequency converter, but this method increases a considerably amount of the installation costs.